1. Field of the Invention
The present invention is directed to a process for reducing the gaseous sulfur-containing products in the gases resulting from combustion of fossil fuels containing sulfur compounds, as well as from other manufacturing/chemical processes. In particular, the present invention is directed to reducing the gaseous sulfur-containing products in the gases by injecting dry powdered alkali hydrates into the gases at a location where their temperature is approximately 800.degree. to 1200.degree. F.
2. Description of the Prior Art
The use of sulfur-bearing fuels in combustion installations (furnaces, burners, boilers, internal combustion reciprocating engines and turbines, and the like) results in the production of sulfur-containing compounds, in particular sulfur dioxide and lesser amounts of sulfur trioxide. Air pollution regulations in many political jurisdictions throughout the world require that these sulfurous constituents be removed from the exhaust gases prior to release into the atmosphere. Various chemical additives are known, including calcium carbonate (limestone), magnesium carbonate (dolomite), and the hydrates of each of these, which react with sulfur dioxide (and sulfur trioxide) in a manner which causes the sulfur compound to be sorbed on these particulate chemicals. It is also known that reactions producing these results can take place at temperatures between approximately 2300.degree. and 1600.degree. F. as well as at temperatures below approximately 350.degree. F. The specific reactions, however, are different in these two temperature regimes, and the reactions in the lower temperature regime (below 350.degree. F.) are greatly enhanced by (1) the presence of water, and/or (2) cooling of the sulfur-containing gases to temperatures approaching their saturation point. The particles formed by the reactions in either temperature regime may be removed from the exhaust gases, with any particles formed during or remaining after combustion, by various means, such as filtration of the flue gases or electrostatic precipitation, and then discarded or recycled.
The efficiency of such a sulfur scrubbing process depends on numerous factors, especially the sorbent used and the temperature environment at, and immediately downstream of, the injection location. The amount of sulfur removed when injected into the higher temperature regime cited above is limited by (1) the rate competition between calcination/sulfation and sintering (disappearance of available reaction surface), and (2) the residence time of the combustion flue gases in the critical temperature window for sulfation (2300.degree. to 1600.degree. F.). In many applications the residence time is too short to achieve more than 15 to 20 percent utilization of the sorbent. Such low utilizations make the economics marginally acceptable for many potential applications.
In addition, rapid, thorough mixing of the injected sorbent with the combustion flue gases is difficult to achieve, especially in large boilers. The most practical and reliable way of introducing the sorbent (particularly at elevated temperatures) is by injecting it through ports in the wall of the combustion volume (see U.S. Pat. No. 3,481,289); with this technique it is difficult to cause the sorbent to penetrate throughout the volume of a big boiler, and large quantities of transport air are required, decreasing the efficiency of the boiler. Attempts to overcome this mixing problem (see U.S. Pat. No. 3,746,498 on premixing the sorbent with the coal prior to introducing them together through the burner; U.S Pat. No. 4,331,638 on injecting the sorbent with the secondary air around the burner; and U.S. Pat. No. 4,440,100 on introducing the sorbent below [the burner] zone) result in deactivation of a substantial portion of the sorbent due to sintering caused by exposure to the high temperatures of the flame and furnace. A process that introduces the sorbent above the burner zone as an aqueous solution or slurry (see U.S. Pat. No. 4,555,996) may overcome the mixing and deactivation (sintering) problem, but increases the complexity of the system, by adding the equipment to prepare and transport the aqueous solution or slurry, and reduces the thermal efficiency of the boiler.
Effective capture of the sulfur compounds when injecting at or below 350.degree. F. requires the development of a system for cooling and/or humidifying the gases without causing wet lime particles or other solid combustion byproducts (e.g., fly ash) to adhere to structures inside the duct or downstream particulate control device. This scheme may also require enlarged ducting to increase the residence time for vaporizing the water droplets (if water injection is used) and, possibly, replacement of the electrostatic precipitator (the most common particulate control system on boilers with no sulfur scrubber) with a baghouse filtration system to provide sufficient time for the reaction. Utilization efficiency of conventional calcitic hydrate sorbents is similar to that achieved by injecting into the higher temperature zone. Hence, the economics of this low temperature process may also be only marginally acceptable for many potential applications.
Therefore, there remains in the art the problem of finding a combination of sorbent, injection location, and injection methodology that (1) provides the correct temperature and residence time to favor a reaction which results in a higher utilization than that obtainable with the high and low temperature processes described above, (2) does not require a high level of humidification, and (3) facilitates the injection/mixing problem in large units.
Various methods have been proposed to attempt to deal with this problem. According to one proposed method, the sorbent is precalcined outside the boiler at conditions which are tailored to produce high specific surface areas and then injected into the combustion flue gases. Utilization was found to increase in proportion to the surface area produced during calcination when commercial sorbents were injected into the higher temperature region (2300.degree. to 1600.degree. F.) of pilot-scale combustors. However, this approach failed to improve sorbent utilization significantly (i.e., by more than 2 to 4 percentage points in most cases) when very high area precalcined sorbents were injected into the higher temperature region. The negative effect of sintering, which depends approximately on the square of the surface area, is greater than the benefit of higher surface area. The technique is being investigated further for use in the lower temperature injection process, but requires that a water humidification system needed for that process be developed.
While the surface area of sorbents might be increased prior to injection by multiple hydration/dehydration steps and/or by hydrating in an ice bath in the presence of alcohol, the resulting sorbents would suffer from the same problem of sintering cited above for high temperature injection and, in any case, this approach may not be effective at lower temperatures.
According to a third known method, an additive such as an alkali metal compound (e.g., sodium), transition element (especially chromium), or iron compound is added to the alkali earth metal sorbent (calcium and/or magnesium carbonate or hydrate), and the mixture injected into the sulfur-containing combustion flue gases. The additive (e.g., an alkali metal) can be physically admixed in a dry state with the sorbent (calcium and/or magnesium compound) or, if the sorbent is a hydrate, incorporated into the hydrate by adding a water soluble compound of the additive to the water of hydration. However, the benefit of this approach, at least on a pilot combustor scale, ranges from none to at most four percentage points increase in sorbent utilization. In addition, many of these additives are expensive, may harm the boiler, and/or may be toxic when discharged into the environment.
It is, therefore, an object of the present invention to provide sorbents and means and methods of sorbent injection that improve the efficiency (speed of reaction and completeness of reaction) of the binding reaction with sulfur-containing gases.